1. Field of the Invention
This invention relates to circuits and methods for recording multiple messages in a semiconductor memory, and particularly relates to marking the boundaries of messages stored in an analog memory.
2. Description of Related Art
Semiconductor memories can record, store, and playback continuous signals such as sound or voice signals. For example, an analog memory can record a voice signal from a microphone by periodically sampling the voice signal and writing samples in a sequence of memory cells. Reading the memory cells in the recorded sequence and sequentially applying voltages read from the memory cells to an amplifier and speaker system plays back the voice signal. As used herein, a message is a signal of limited duration that may be stored in a semiconductor memory. The amount of memory (i.e., the number of memory cells) required to store a message depends upon the duration of the message and the sampling frequency used when recording the message. Current memories have sufficient capacity to store multiple short messages in a single memory chip or array. However, memory systems that store multiple messages must identify the boundaries of individual messages so that a selected message can be played back without sequentially reading through all messages or can be erased without disturbing other messages.
One method for identifying the boundaries of messages in a memory uses special memory cells that are not used for recording samples but instead are set to identify the end of a message. For example, in a recording system that sequentially writes samples in rows of a memory array, the last memory cell in each row can be reserved for storing an end-of-message (EOM) mark. At the end of a message, the last memory cell in the last row containing samples for the message is programmed to create the EOM mark for the message. The EOM marks identify messages for playback or erase and identify memory that is available for recording of new messages.
The above described system for marking the boundaries of messages in a memory is not well suited for Flash memories. In Flash memories, entire sectors of memory cells are simultaneously erased. If the memory cells reserved for EOM marks are part of a sector containing samples, the EOM marks are erased each time the sector is erased. However, such cells may be rarely programmed, i.e. only when an associated row is the last row of a message. Accordingly, the EOM cells dedicated to marking the ends of messages may be repeatedly erased without being programmed and can become overerased. The overerased cells can disrupt operation of the memory for example, by conducting even when not selected during a read or programming. Accordingly, a system and process for recording multiple messages is sought that resists overerasing of memory cells and is better suited to Flash memory.